1. Field of the Invention
The present invention relates to a catalyst suitably used for purification of exhaust gases, preferably nitrogen oxides emitted from internal combustion engines, etc.
2. Description of the Prior Art
Removal of air pollutants is an important task in view of the protection of global environment. Removal of, in particular, nitrogen oxides which are said to be a cause for photochemical smog and acid rain, is a task requiring an urgent solution.
Selective catalytic reduction using ammonia has hitherto been used for exhaust gases emitted from large-sized stationary sources such as thermal power stations and the like, and a considerable result has been obtained for purification of said exhaust gases.
Meanwhile, for exhaust gases emitted from on-site cogeneration engines or gasoline engines of passenger cars, trucks, etc., there have been used three-way catalysts (TWCs) capable of simultaneously removing carbon monoxide (CO), hydrocarbons (HCs) and nitrogen oxides (NOx) with the air-fuel ratio being controlled to about a stoichiometric level (A/F=14.6).
In recent years, the control of carbon dioxide (CO.sub.2) emission has become necessary for the protection of global warming, and the practical application of lean-burn engines has been desired. Three-way catalysts, however, are not effective for said lean-burn engines.
Diesel engines, which are basically a lean-burn engine, must use a means such as exhaust gas recirculation (EGR), delayed fuel injection timing or the like in order to control the NOx in exhaust gas, and the presence of a certain amount of suspended particulates in exhaust gas has been allowed. In the future, however, stricter regulation will be imposed on both the particulates and NOx emitted from diesel engines. Incidentally, in the exhaust gases from diesel engines, the amounts of particulates and NOx are in a trade-off relationship, and it is considered that the amount of particulates can be reduced significantly in the NOx can be efficiently removed with a catalyst.
Engines employing lean burn system, such as lean-burn gasoline engines, diesel engines and the like are hereinafter referred to generically as lean-burn engines.
In recent years, various catalyst systems have been proposed for the purpose of abatement of NOx in exhaust gases emitted from lean-burn engines, containing oxygen in excess of stoichiometric amount. For example, a copper ion-exchanged zeolite was disclosed in U.S. Pat. No. 4,297,328 and Japanese Patent Application Kokai (Laid-Open) No. 100919/1988; a noble metal (e.g. platinum, palladium, rhodium)-ion-exchanged zeolite was disclosed in Japanese Patent Application Kokai (Laid-Open) No. 135541/1989; a copper ion-exchanged metallosilicate was disclosed in Japanese Patent Application Kokai (Laid-Open) No. 127628/1991; and a copper and VIII group element-on-metallosilicate catalyst was disclosed in Japanese Patent Application Kokai (Laid-Open) No. 229620/1991.
These conventional zeolite or metallosilicate type catalyst [hereinafter referred to generically as metallosilicate type catalysts], however, have detrimental drawbacks in, for example, that the temperature range effective for NOx abatement is limited and that the activity deterioration is remarkable in high-temperature exhaust gases containing steam. Hence, their performances are not yet at a level allowing practical application.